Automatic voltage regulating system for engine generator

ABSTRACT

An automatic voltage-regulating system for an engine generator includes a rectifying circuit connected to an exciting winding, a smoothing condenser provided between plus-side and minus-side output terminals of the rectifying circuit, an output voltage detecting means for extracting a voltage generated in output windings to output a DC detection voltage, and a switching means provided between the other end of the field winding and the minus-side output terminal so as to enable switching between the break and conducting one from another in depending on a voltage detected by the output voltage detecting device. In this automatic voltage regulating system, the charged voltage in the smoothing condenser is divided by a pair of voltage dividing resistors, and the output terminal of the output voltage detecting device is forcibly and electrically connected to the minus-side DC output terminal of the rectifying circuit by a transistor in response to the voltage at a connection point between both the voltage dividing resistors exceeding a predetermined voltage. Thus, by adding a simple circuit constructed of a reduced number of parts or components, the fracture of the smoothing condenser can be prevented despite the provision of a condensive load, thereby providing a stable output voltage.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an automatic voltage-regulating system for an engine generator, comprising a rectifying circuit which has plus-side and minus-side DC output terminals and which is connected to an exciting winding with its plus-side DC output terminal connected to one end of a field winding, a smoothing condenser provided between both the DC output terminals of the rectifying circuit, an output voltage detecting means for extracting a voltage generated in output windings to output a DC detection voltage, and a switching means provided between the other end of the field winding and the minus-side output terminal so as to enable switching between the break when the voltage provided by the output voltage detecting means is high and the conducting when the voltage provided by the output voltage detecting means is low, wherein the field current to the field winding based on the voltage generated in the exciting winding is controlled to regularize the voltage output from the output windings.

[0003] 2. Description of the Related Art

[0004] A prior art automatic voltage-regulating system for an engine generator is designed such that the field current flowing in a field winding 3 is controlled based on a voltage generated in an exciting winding 4 by turning on and off a switching means 10 including a plurality of transistors 22, 23 and 24 connected through the Darlington circuit by a DC detecting voltage provided by extracting a portion of the voltage generated in output windings 1 and 2, as shown in FIG. 3, thereby regularizing the voltage generated in the output windings 1 and 2. However, if a load 6 connected to either one of the output windings 1 and 2, e.g., to the output winding 1 is a condensive load such as a mercury-arc lamp, a metal halide lamp and the like, there is a possibility that the fracture of the smoothing condenser 8 may occur based on an induced voltage generated in the field winding 3.

[0005] More specifically, if the load 6 is a condensive load, an alternating voltage of an even-number order higher harmonic is induced in the field winding 3 by flowing of a condensive current in the output winding 1. If one cycle of an alternating voltage of a low-order higher harmonic is considered, electric current, in a minus-side half-wave, flows from a negative electrode 3 m of the field winding 3 via a diode 12 to a positive electrode 3 p of the field winding 3, as shown by broken lines, and on the other hand, in a plus-side half-wave, electric current flows from the positive electrode 3 p of the field winding 3 via the smoothing condenser 8 and a diode 13 to the negative electrode 3 m of the field winding 3, as shown by solid lines. Therefore, the smoothing condenser 8 is charged with an induced voltage produced in the field winding 3, and there is a possibility of fracturing of the smoothing condenser 8 due to a voltage of a rated value or more being applied to the smoothing condenser 8.

[0006] To solve the above problem, an automatic voltage-regulating system disclosed in Japanese Utility Model Laid-open No.4-101300 is designed such that the fracture of the smoothing condenser 8 is prevented by the following two circuits: a short circuit which is connected in parallel to the field winding 3 to short-circuit an induced voltage produced in the field winding 3, when the induced voltage exceeds a predetermined value, and an inhibiting circuit comprising a diode connected in series to the field winding 3 in a direction of the flowing of the field current.

[0007] In the automatic voltage-regulating system disclosed in the above Japanese Utility Model Laid-open No.4-101300, however, the short circuit including a thyristor and the inhibiting circuit including the diode are required. Thus, it is impossible to mention that number of parts is small as a circuit arrangement for preventing the fracture of the smoothing condenser in the case the condensive load is connected to the output winding, or to mention that the circuit arrangement is simple.

SUMMARY OF THE INVENTION

[0008] Accordingly, it is an object of the present invention to provide an automatic voltage-regulating system of the above-described type for an engine generator, wherein the fracture of the smoothing condenser can be prevented despite provision of the condensive load, by additionally providing a simple circuit constructed of a reduced number of parts, thereby providing a stable output voltage.

[0009] To achieve the above object, according to the present invention, there is provided an automatic voltage-regulating system for an engine generator, comprising a rectifying circuit which has plus-side and minus-side DC output terminals and which is connected to an exciting winding with its plus-side DC output terminal connected to one end of a field winding, a smoothing condenser provided between both the DC output terminals of the rectifying circuit, an output voltage detecting means for extracting a voltage generated in an output winding to output a DC detection voltage from an output terminal, and a switching means provided between the other end of the field winding and the minus-side DC output terminal so as to enable switching a break when the voltage provided by the output voltage detecting means is high and a conducting when the voltage provided by the output voltage detecting means is low, so that a field current to the field winding based on the voltage generated in the exciting winding is controlled to regularize the voltage output from the output windings, wherein the system further includes a pair of voltage dividing resistors for dividing a charged voltage in the smoothing condenser, and a transistor for forcibly and electrically connecting the output terminal of the output voltage detecting means to the minus-side DC output terminal of the rectifying circuit in response to the voltage at a connection point between both the voltage dividing resistors exceeding a predetermined voltage.

[0010] With such arrangement, if an induced voltage is produced in the field winding in response to flowing of condensive current in the output winding due to the connection of a condensive load, the smoothing condenser is charged. If the charged voltage in the smoothing condenser becomes high and the voltage at the connection point between both the voltage dividing resistors exceeds the predetermined voltage, the output terminal of the output voltage detecting means is forcibly and electrically connected to the minus-side DC terminal of the rectifying circuit by the transistor. Therefore, the switching means electrically connects the other end of the field winding to the minus-side DC output terminal of the rectifying circuit, irrespective of the detection voltage detected by the output voltage detecting means, whereby the charged voltage in the smoothing condenser is discharged via the field winding and the switching means. Thus, it is possible to prevent the smoothing condenser from being fractured due to the charging of a voltage of a rated value or more. Moreover, to prevent the fracture of the smoothing condenser, it is only required to add, to the automatic voltage regulating system, a circuit which is simple and constructed of reduced number of parts including a pair of voltage dividing resistors and a transistor.

[0011] The above and other objects, features and advantages of the invention will become apparent from the following description of the preferred embodiments taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 shows a circuit arrangement of an automatic voltage regulating system according to a first embodiment of the present invention;

[0013]FIG. 2 shows a circuit arrangement of an automatic voltage regulating system according to a second embodiment of the present invention; and

[0014]FIG. 3 shows a circuit arrangement of the prior art automatic voltage regulating system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] A first embodiment of the present invention will now be described with reference to FIG. 1. An engine generator includes a pair of output windings 1 and 2, a field winding 3 wound around a rotor which is not shown, an exciting winding 4 for generating a voltage for permitting a field current to flow in the field winding 3, and an automatic voltage-regulating device 5 ₁. A condensive load 6 such as a mercury lamp is connected to at least one of the output windings 1 and 2, e.g., the output winding 1.

[0016] The automatic voltage-regulating device 5 ₁ includes a rectifying circuit 7 connected to the exciting winding 4, a smoothing condenser 8 for smoothing the DC output provided in the rectifying circuit 7, an output voltage detecting means 9 for extracting a portion of the voltage generated in the output windings 1 and 2 to provide a DC detection voltage, a switching means 10 adapted to be turned on or off depending on the voltage provided by the output voltage detecting means 9 to control the field current to the field winding 3, and a condensive load countermeasure means 11 applied according to the present invention to prevent the rupture of the smoothing condenser 8 based on an induction voltage induced in the output windings 1 and 2 due to the condensive load 6.

[0017] The rectifying circuit 7 has a plus-side DC output terminal 7 p and a minus-side DC output terminal 7 m and is connected to the exciting winding 4. The plus-side DC output terminal 7 p is connected to a positive electrode 3 p of the field winding 3 through a plus-side line Lp, and the minus-side DC output terminal 7 m is connected to a minus-side line Lm.

[0018] The smoothing condenser 8 is mounted between both the DC output terminals 7 p and 7 m of the rectifying circuit 7, i.e., between the plus-side and minus-side lines Lp and Lm. A diode 12 is mounted between the negative electrode 3 m and the positive electrode 3 p of the field winding 3 for limiting the flow of electric current from the negative electrode 3 m to the positive electrode 3 p, and a diode 13 is mounted between the minus-side line Lm and the negative electrode 3 m for limiting the flow of electric current from the minus-side line Lm to the negative electrode 3 m.

[0019] The output voltage detecting means 9 for extracting the voltage output from the output winding 1 to provide a DC detection voltage, and includes a rectifying circuit 14 connected to the output winding 1 and having a plus-side DC output terminal 14 p and a minus-side DC output terminal 14 m connected to the minus-side line Lm, a resistor 15 and a smoothing condenser 16 connected in series between the plus-side output terminal 14 p and the minus-side line Lm, resistors 17, 18 and 19 connected in series between a connection point between the resistor 15 and the smoothing condenser 16 and the minus-side line Lm, and a variable resistor 20 connected in parallel to the resistor 19, so that the DC detection current provided by extracting the voltage generated in the output winding 1 is output from an output terminal 21 which is a connection point between the resistors 17 and 18. When the output voltage from the output winding 1 is high, the DC detection voltage may be provided through an intermediate tap so as to extract a portion of the output voltage.

[0020] The switching means 10 includes three npn transistors 22, 23 and 24 which are, for example, connected by the Darlington circuit to one another. The transistor 22 is provided between the negative electrode 3 m of the field winding 3 and the minus-side line Lm; the transistor 23 is provided between the negative electrode 3 m of the field winding 3 and the base of the transistor 22; and the transistor 24 is provided between the base of the transistor 23 and the minus-side line Lm. The base of the transistor 23 is connected to the plus-side line Lp through a resistor 25, and a diode 26 is provided between an emitter and a base of the transistor 23.

[0021] The base of the transistor 24 is connected to the minus-side line Lm through a resistor 27 and also connected the negative electrode 3 m of the field winding 3 through a capacitor 28 and a resistor 29, and a capacitor 30 is provided between the bases of the transistors 23 and 24. Further, the base of the transistor 24 is connected to the output terminal 21 of the output voltage detecting means 9 through a Zener diode 31 for regularizing the voltage input to the base.

[0022] The above switching means 10 ensures that when the output voltage from the output terminal 21 in the output voltage detecting means 9 is high, the transistor 22 is cut off in response to the conduction of the transistor 24 to electrically disconnect the negative electrode 3 m of the field winding 3 and the minus-side DC output terminal 7 m of the rectifying circuit 7. On the other hand, when the output voltage from the output terminal 21 in the output voltage detecting means 9 is low, the transistor 22 is allowed to conduct in response to the cutting-off of the transistor 24 to electrically connect the negative electrode 3 m of the field winding 3 and the minus-side DC output terminal 7 m of the rectifying circuit 7. Thus, the field current to the field winding 3 based on the voltage generated in the exciting winding 4 is controlled by the switching operation of the switching means 10, whereby the output voltage from the output windings 1 and 2 is regularized.

[0023] The condensive load countermeasure means 11 includes a pair of voltage dividing resistors 32 and 33 connected in series between the plus-side line Lp and the minus-side line Lm, i.e., between opposite ends of the smoothing condenser 8, an npn transistor 34 provided between the output terminal 21 of the output voltage detecting means 9 and the minus-side line Lm, i.e., the minus-side DC output terminal 7 m of the rectifying circuit 7, a diode 35 provided between a connection point between the voltage dividing resistors 32 and 33 and the base of the transistor 34, and a capacitor 36 provided between the base of the transistor 34 and the minus-side line Lm.

[0024] The condensive load countermeasure means 11 ensures that when the voltage provided by dividing the voltage between the plus-side and minus-side lines Lp and Lm by the voltage dividing resistors 32 and 33 exceeds a predetermined voltage, the transistor 34 is allowed to conduct, whereby the output terminal 21 of the output voltage detecting means 9 is forcibly electrically connected to the minus-side line Lm, i.e., the minus-side DC output terminals 7 m and 14 m of the rectifying circuits 7 and 14, and the transistor 24 in the switching means 10 is cut off.

[0025] In the condensive load countermeasure means 11, the diode 35 is used for regularizing the base voltage in the transistor 34. To obtain such a function, it is common to use a Zener diode. In the switching means 10, however, the plurality of transistors 22, 23 and 24 are connected by the Darlington circuit with one another and hence, the base current in the transistor 24, i.e., the current flowing from the output terminal of the output voltage detecting means 9 may be extremely small. Therefore, the capacitor of the transistor 34 provided between the output terminal 21 of the output voltage detecting means 9 and the minus-side line Lm may be also extremely small. Thus, to regularize the base voltage of the transistor 34, a forward band offset of the diode 35 can be used without use of a backward band offset of a Zener diode. The Zener diode is generally encapsulated in a glass pipe and hence, the greatest care must be taken in the handling during assembling, and an increase in cost is brought about. On the contrast, the use of the diode 35, as described above, facilitates the handling during assembling, but also contributes to a reduction in cost.

[0026] Further, the capacitor 36 acts to prevent the fracture of the transistor 22 due to the chattering of the transistor 34, and also acts to prevent the mis-operation of the transistor 34 due to a surge voltage.

[0027] The operation of the first embodiment will be described below. If a condensive current flows across the output winding 1 due to the connection of the condensive load 6 thereto, an alternating voltage of an even-number order higher harmonic is induced in the field winding 3. In one cycle of the alternating voltage, in a minus-side half-wave in, electric current flows from the negative electrode 3 m of the field winding 3 via the diode 12 to the positive electrode 3 p of the field winding 3, as shown by broken lines in FIG. 3, and in a plus-side half-wave, electric current flows from positive electrode 3 p of the field winding 3 via the smoothing condenser 8 and the diode 13 to the negative electrode 3 m of the field winding 3, whereby the smoothing dondenser 8 is charged with the induced voltage generated in the field winding 3.

[0028] However, the condensive load countermeasure means 11 added to the automatic voltage-regulating device 5 ₁ includes the pair of voltage dividing resistors 32 and 33 for dividing the charged voltage in the smoothing condenser 8, and the transistor 34 for forcibly and electrically connecting the output terminal 21 of the output voltage detecting means 9 to the minus-side DC output terminals 7 m and 14 m of the rectifying circuits 7 and 14 in response to the voltage at the connection point between the voltage dividing resistors 32 and 33 exceeding the predetermined voltage. Therefore, when the charged voltage in the smoothing condenser 8 is risen, so that the voltage at the connection point between the voltage dividing resistors 32 and 33 exceeds the predetermined voltage, the switching means 10 electrically connects the negative electrode 3 _(m) of the field winding 3 to the minus-side DC output terminal 7 _(m) of the rectifying circuit 7 in response to the conduction of the transistor 34, irrespective of the detection voltage detected by the output voltage detecting means 9. Therefore, when the voltage generated in the field winding 3 is lower than the charged voltage in the smoothing condenser 8, the charged voltage in the smoothing condenser 8 is discharged through the positive electrode 3 p and the negative electrode 3 _(m) of the field winding 3 and through the transistor 22. Thus, it is possible to prevent the smoothing condenser 8 from being fractured due to the charging of a voltage of a rated value or more.

[0029] Moreover, to prevent the fracture of the smoothing condenser 8, it is only required to add the condensive load countermeasure means 11 including the pair of voltage dividing resistors 32 and 33 and the transistor 34 and having a simple circuit arrangement comprising a reduced number of components.

[0030]FIG. 2 shows a second embodiment of the present invention. An automatic voltage-regulating device 5 ₂ according to the second embodiment is used when the output from an engien generator is larger. The automatic voltage-regulating device 5 ₂ is different from the automatic voltage-regulating device 5 ₁ only in respect of a switching means 10′, and the other construction is the same as in the automatic voltage-regulating device 5 ₁ according to the first embodiment.

[0031] The switching means 10′ is different from the switching means 10 in the first embodiment in respect of a pair of transistors 22, 22 being provided in parallel between the negative electrode 3 m of the field winding 3 and the minus-side line Lm.

[0032] Even with the second embodiment, even if the smoothing condenser 8 is charged due to the connection of the condensive load 6 thereto, the smoothing condenser 8 can be prevented from being fractured due to the charging with a voltage of a rated value or more.

[0033] Although the embodiments of the present invention have been described in detail, it will be understood that the present invention is not limited to the above-described embodiments, and various modifications may be made without departing from the spirit and scope of the invention defined in claims. 

What is claimed is:
 1. An automatic voltage-regulating system for an engine generator, comprising a rectifying circuit which has plus-side and minus-side DC output terminals and which is connected to an exciting winding with said plus-side DC output terminal connected to one end of a field winding, a smoothing condenser provided between both said DC output terminals of said rectifying circuit, an output voltage detecting means for extracting a voltage generated in an output winding to output a DC detection voltage from an output terminal, and a switching means provided between the other end of said field winding and said minus-side DC output terminal so as to enable switching between a break when the voltage provided by said output voltage detecting means is high and a conducting when the voltage provided by said output voltage detecting means is low, so that a field current to said field winding based on the voltage generated in said exciting winding is controlled to regularize the voltage output from said output windings, wherein the system further includes a pair of voltage dividing resistors for dividing a charged voltage in said smoothing condenser, and a transistor for forcibly and electrically connecting the output terminal of said output voltage detecting means to the minus-side DC output terminal of said rectifying circuit in response to the voltage at a connection point between both the voltage dividing resistors exceeding a predetermined voltage. 